Collaborative Proposal: Multiscale Modeling of Damage Tolerance in Hexagonal Materials
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Hexagonal close-packed crystalline (hexagonal) metals are widely employed in industrial and biomedical sectors, e.g. as fuel rods in nuclear power plants (zirconium alloys); medical stents and dental applications (titanium and magnesium alloys); compressor and turbine disks and blades in power generation systems and jet engines (titanium alloys) and in cryogenic fuel tanks and space telescope mirrors (beryllium alloys) among many others. Yet, there currently are no material modeling frameworks that can be used to make robust engineering projections regarding their durability and damage tolerance. This award supports fundamental research addressing this critical gap and focus is laid on predictive modeling of strength and ductility limits. The outcomes of this research will not only advance life assessment procedures but also avoid material waste in processing and manufacturing operations. The models and simulation tools to be developed will provide engineers with means of predicting the mechanical response of metallic structures under complex loading conditions, both during manufacturing and in service. In addition, fundamental understanding gained from this research will help alloy and microstructure designers develop damage-tolerant materials by defeating the controlling mechanisms rather than following trial-and-error approaches. In addition, the PIs will engage undergraduate students in this research program and develop lectures and accompanying material for a summer school aimed at empowering the next generation of mechanical engineers with computational mechanics and materials science tools. Improved understanding of the slip and twinning mechanisms in hexagonal metals has led to advances in constitutive modeling of their damage-free plasticity. However, micromechanics-based models that couple plasticity with damage remain to be developed. One challenge is to assess the extent of crystallographic detail that must be incorporated in a damage model. Another, equally important issue is one of representation of the damage process. Recent experiments have clearly shown that hexagonal alloys do fail by void nucleation, growth and coalescence. With the goal of a parameter-free formulation of damage and fracture, this collaborative research aims at fundamental investigations of ductile damage in hexagonal materials under general stress states by - (i) investigating the role of deformation mechanisms on micro-void growth and coalescence using three-dimensional crystal plasticity unit cell simulations, (ii) formulating a novel, computationally efficient micromechanics-based anisotropic porous multi-surface model, fully assessed against the unit cell calculations, and (iii) critically assessing the so-developed anisotropic damage model against existing experimental data for magnesium alloys. This award reflects NSF''s statutory mission and has been deemed worthy of support through evaluation using the Foundation''s intellectual merit and broader impacts review criteria.